1. Field of the Invention
The present invention relates to method and apparatus for self-control in distributed priority collision wherein a plurality of units sharing a common unit are self controlled to prevent collision of access requests from the plurality of units.
For example, a system in which information is transmitted by a central processing unit and a plurality of terminal devices through a common bus line and a system in which a plurality of computers share a common memory are applicable. While such a system usually has a common bus line, the present invention is not limited to such a system. For example, the present invention is applicable to a system in which a common unit and a plurality of terminal devices are connected via respective information transmission lines but the common unit cannot be simultaneously accessed by the plurality of terminal devices.
2. Description of the Prior Art
A problem of collision is usually encountered when a common unit such as a common central processing unit is shared by a plurality of terminal devices. Many methods to resolve the above problem have been proposed.
In one method, a priority control unit is provided to control access requests from the plurality of terminal devices. The control unit receives access request signals from the plurality of terminal devices and issues access grant signals to the plurality of terminal devices in accordance with a predetermined priority order. Only the terminal device which received the access grant signal can access to the common unit (e.g. central processing unit). However, the priority control unit and signal lines for the access request signals and the access grant signals are additionally needed, and if an error occurs in the priority control unit, all of the plurality of terminal devices are affected.
In another method, a plurality of terminal devices are connected in a closed loop and a right to access a central processing unit is sequentially shifted. If a terminal device issues the access request to the central processing unit when that terminal device possesses the access right, it is permitted to access the central processing unit. This method is called a round robin method or a daisy chain method. In this method, however, if failure occurs in one of the terminal devices, all of the downstream terminal devices (and hence all terminal devices) are affected.
Japanese Laid-Open Patent Application No. 55-23643 (laid-open on Feb. 20, 1980) discloses a method in which specific addresses are assigned to a plurality of terminal devices and an inhibit signal for inhibiting the send-out of data from other terminal devices is sent out to an inhibit line at a timing associated with the addresses. However, this method needs the inhibit line in addition to the data lines.
U.S. Pat. No. 4,063,220 entitled "Multipoint Data Communication System with Collision Detection" issued on Dec. 13, 1977 discloses a method in which when terminal devices request data transmission, they are allowed to transmit data and a collision detector detects any collision, and a time interval to the next transmission is determined by a random number generator. This method is characterized by handling the collision after the terminal devices were allowed to transmit the data.